Infusion pumps are used for infusing fluids, which can include drugs or nutrients, into circulatory systems of humans or animals. Infusion pumps can be of various types, such as syringe infusion pumps and volumetric infusion pumps. Volumetric infusion pumps function by dispensing fluid from an intravenous (IV) fluid bag (also referred in as an IV bag or IV container) suspended at a head height relative to the patient. Mechanisms for regulating fluid flow from an IV bag can include, for example, rotary and linear peristaltic mechanisms as well as piston driven cartridge mechanisms. Such mechanisms can include a motor to drive the pumping element in an active fashion to provide a fluid flow from the bag. In the case of a rotary peristaltic mechanism, a motor can be used to cause a wheel with protrusions to rotate in contact with the tubing, causing fluid to be pushed through the tube. In the case of a linear peristaltic mechanism, a motor can be used to turn a crankshaft or camshaft which pushes fingers into the tubing in a peristaltic wave, thereby causing the fluid flow. In the case of a piston cartridge mechanism, a motor can be used to cycle the piston back and forth and in conjunction with two or more check valves to cause fluid flow.
In the case of peristaltic pumps, the fluid tubing is located with respect to the pumping elements such that sufficient closure of the tubing is provided to ensure pumping action when fluid flow is desired and full closure of the tubing when the pump is stopped. To allow a user to install a new tubing set for each new therapy, a door is typically used to function both as a platen which supports the tubing as the fingers pump against it, as well as to ensure that the tubing remains in communication with the pumping fingers such that the tube is always compressed at one or more points. In the event that the door does not close properly or the tubing is not fully closed by one or more of the fingers, free flow of fluid due to gravity can occur resulting in an overdose of medication to the patient. In addition, the fluid flow rate can be computed based on the speed of the motor. The motor is set to a particular RPM, which by the nature of the cross sectional area of the tubing and the gearing of the mechanism corresponds to an expected fluid flow. Since the pump measures only motor RPM and not actual fluid flow, errors can be caused if the tubing cross sectional area is less than what is expected due to, for example, non-elastic deformation of the tubing. Also, the precise location of the tubing set in relation to the mechanism requires precision bearings and other mechanical features, which can add cost and complexity and can detract from the reliability of the pumps.
In addition, such positive pressure pumps pose the hazard of pumping air into the patient, potentially causing air embolism. Even though sensors can be added to detect the presence of air in the tube set, the sensors are subject to false alarms as well as reliability issues. Typical medical fluid infusion pumps also can be subject to difficulties in accurately sensing the exhaustion of fluid from the IV bag, or upstream or downstream occlusion.